This is an exploratory project to investigate by non-invasive methods basic biological responses of experimentally induced astrocytomas in brain. The redox state of NADH and of cytochromes, the oxygenation of hemoglobin, the electric potential, and the activity of potassium ions will be measured in situ in tumor tissue and in normal cerebral cortex, when challenged by hypoxia and hyperoxia, and by variation of potassium levels in fluid superfusing the cerebral surface.